Three seemingly very different people with one thing in common: All three were accused of sexually assaulting fellow passengers on airplanes.

Even before Jessica Leeds alleged last week that Donald Trump touched her inappropriately during a flight in 1979, many frequent fliers had concluded that increasingly cramped planes with fewer flight attendants walking the aisle seem to embolden gropers.

“Sexual harassment and assault is happening on aircraft, and we believe it’s happening more often because of the conditions on board,” said Sara Nelson, the international president of the Association of Flight Attendants-CWA union. She cited cramped, confined spaces; alcohol and drugs; fewer flight attendants; and dark cabins on night flights as factors that likely embolden offenders.

Prosecutors said that the Rev. Marcelo De Jesumaria testified that he considered his touching his sleeping victim on a US Airways flight from Philadelphia to Los Angeles in 2014 “consensual because she did not reject his touches and he interpreted her silence, because she was asleep, as ‘coyness.’ ”

When Mr. De Jesumaria relaxed his grip, the victim went to the bathroom and used the call button to summon a flight attendant.

The flight crew reseated him between two male passengers, and law enforcement was waiting when the plane landed in Los Angeles.

Mr. De Jesumaria, 47, who previously served in the Roman Catholic Diocese of San Bernardino, Calif., was sentenced to six months in prison and six months of home confinement after being convicted of abusive sexual contact.

Mr. De Jesumaria had not been seated next to his victim initially, but switched seats by asking a flight attendant if he could “sit next to his wife.”

Heidi Anne McKinney, 26, was charged with touching another woman on the thigh and groin during an Alaska Airlines flight from Las Vegas to Portland on May 8 this year.

In another case, according to a criminal complaint filed in United States District Court in New Jersey, a woman allegedly assaulted by Yoel Oberlander on an overnight El Al flight from Tel Aviv to Newark on May 29 was seated between him and her mother when he began to grope her. She kept repositioning herself to shake his hand off her hand, thigh and breast. It wasn’t until her mother awoke that she asked her to switch seats, and eventually reported to the crew what had taken place.

Mr. Oberlander, 35, was charged with one count of abusive sexual contact on an airplane. He is a registered sex offender convicted in 2002 of sexually assaulting an 11-year-old girl in New York.

Just how frequent sexual assault is during air travel is difficult to determine, but F.B.I. investigations into in-flight sexual assaults have increased 45 percent so far this year. The bureau said that it had opened 58 investigations into sexual assault on aircraft from January through September 2016, compared with 40 for all of 2015. That increase doesn’t include incidents reported to local and airport police. It also doesn’t account for the 75 percent of sexual assaults that generally go unreported, according to the Bureau of Justice Statistics, a division of the Department of Justice.

Among the airborne sexual assaults reported to the F.B.I. this year was that of a 13-year-old unaccompanied minor who may have been touched inappropriately by a man who had been drinking at the Dallas airport before boarding an American Airlines flight to Portland, Ore., and a woman who said she awoke on a Virgin America redeye flight from Los Angeles to Newark to find the man next to her massaging her genitals and rubbing his bare feet against her.

There is no centralized system for collecting sexual assault reports from airlines, and no special training for flight attendants in handling sexual assault.

“This is a unique crime,” said Ms. Nelson, who in addition to her union position is a United Airlines flight attendant with 20 years of experience. “It’s really not the same as asking, ‘How much did that person hurt you when they hit you on the head?’ ”

Unless police are called to meet the flight, it is up to the crew to decide whether to report disruptive behavior to the Federal Aviation Administration. When disturbances are reported, there is no separate category for sexual assault.

Photo

Jessica Leeds, 74, who said that Donald J. Trump touched her inappropriately on an airplane in the early 1980s.Credit
George Etheredge for The New York Times

“It’s one thing to talk about the alertness to security concerns, but this is a crime that has not even been specifically identified” by the airlines, Ms. Nelson said.

An American Airlines spokesman, Ross Feinstein, said that it is not up to the crew to assess whether a crime, or what type of crime, occurred.

“We’re reporting misconduct that occurred on the aircraft. It’s up to law enforcement to determine if any criminal misconduct occurred,” he said. Regardless of the situation, all conflicts on aircraft are handled the same way by separating those involved, deciding if a diversion of the plane is necessary, and calling ahead for law enforcement to meet it.

An error has occurred. Please try again later.

You are already subscribed to this email.

But the lack of data on airplane sexual assault makes it difficult to study.

“It’s hard to assess what’s going on if we don’t know the extent of what’s happening,” said Elizabeth L. Jeglic, an associate professor specializing in sex offender policy and treatment at John Jay College of Criminal Justice in New York City. She said she did not know of any studies on airplane sexual assaults.

Still, with about 712 million passengers on United States flights in the last year, the number of passengers who are sexually assaulted is a tiny percentage of overall air travelers.

Unruly passenger behavior has been increasing worldwide, jumping 17 percent from 2014 to 2015, according to numbers reported to the International Air Transport Association by its 265 member airlines.

Alcohol or drugs were identified as a factor in 23 percent of the 10,854 disruptive incidents last year, the trade association said.

Those who commit sexual violence use alcohol to exploit their victims’ vulnerability and to lower their own inhibitions, said Laura Palumbo, communications director for the National Sexual Violence Resource Center.

“The third thing that people count on when alcohol is involved is that it will excuse their own actions,” she said.

Crew members already receive training on serving alcohol responsibly. The Air Transport Association is now calling on airport bars and duty-free shops to voluntarily follow suit so that passengers aren’t drunk when they board the plane.

Ms. Palumbo said that there were other factors involved in sexual assaults as well.

“You don’t necessarily get to choose what your physical boundaries are from the people around you because of the nature of transportation,” she said.

“You have the close proximity, and with the proximity there is forced intimacy,” Dr. Jeglic said.

There also are fewer flight attendants on planes to keep an eye on what’s happening between the rows. Although the Federal Aviation Administration specifies minimum crew staffing for each type of aircraft based on evacuation times, airline cutbacks in the travel downturn following Sept. 11 eliminated some flight attendants, according to a study by Diane L. Damos published in the International Journal of Aviation Psychology in 2013.

“If there were more flight attendants who were able to monitor the cabin and trained in what signs to look for, and we were actually able to identify this as a potential threat on board the aircraft, we might be able to better address this problem,” Ms. Nelson said.

The flight attendants’ union has been working with members of Congress and victim advocacy groups on legislation that would expand crew training to include dealing with victims of sexual assault on a flight, as well as to create new industry reporting standards. She said it was too soon to provide specifics.

An F.A.A. spokeswoman said the agency would not comment on pending legislation.

An earlier effort by Rep. Eleanor Holmes Norton, Democrat from the District of Columbia, failed in 2014. Her bill, “The Protecting Airline Passengers From Sexual Assaults Act,” would have required the F.A.A. to collect and publish data on sexual assault. Her office did not return a phone call seeking comment.

The National Sexual Violence Resource Center, which was involved in helping the Transportation Safety Administration change its passenger screening guidelines, has not been involved with the legislative efforts, but its spokeswoman said that, based on other research, more can be done to address airline sexual assault.

“There is a strong body of research that lets us know when people are given the tools to understand what sexual violence is, how best to intervene in instances of sexual violence, and have training and policies as well as those steps, it can lower rates of sexual violence and can be in the best interest of passenger safety,” Ms. Palumbo said.

A version of this article appears in print on October 30, 2016, on Page TR4 of the New York edition with the headline: New Attention on Sexual Assault in Airplanes. Order Reprints|Today's Paper|Subscribe